One of the most demanding applications of materials in aircraft gas turbine engines is compressor and fan disks (sometimes termed “rotors”) upon which the respective compressor blades and fan blades are supported. The disks rotate at many thousands of revolutions per minute, in a moderately elevated-temperature environment, when the gas turbine is operating. They must exhibit the required mechanical properties under these operating conditions.
Some of the gas turbine engine components, such as some of the compressor and fan disks, are fabricated from titanium metallic compositions. The disks are typically manufactured by furnishing the metallic constituents of the selected titanium metallic composition, melting the constituents, and casting an ingot of the titanium metallic composition. The cast ingot is then converted into a billet. The billet is further mechanically worked, typically by forging. The worked billet is thereafter upset forged, and then machined to produce the titanium-base metallic composition component.
Achieving the required mechanical properties at room temperature and up to moderately elevated temperatures, retaining sufficient environmental resistance, and preventing premature failure offer major challenges in the selection of the materials of construction and the fabrication of the articles. The chemistry and microstructure of the metallic composition must ensure that the mechanical properties of the article are met over the service temperature range of at least up to about 1200° F. for current titanium-base metallic composition components. The upper limit of about 1200° F. for service of such components is due principally to static-strength and creep-strength reduction at higher temperatures and the tendency for titanium to react with oxygen at elevated temperatures, forming a brittle oxygen-enriched layer, termed alpha case. Small mechanical or chemical irregularities in the final component may cause it to fail prematurely in service, and these irregularities must be minimized or, if present, be detectable by available inspection techniques and taken into account. Such irregularities may include, for example, mechanical irregularities such as cracks and voids, and chemical irregularities such as hard alpha irregularities (sometimes termed low-density inclusions) and high-density inclusions.
One recent approach to improving the properties of titanium-base metallic compositions, including the high-temperature strength, is the introduction of boron into the metallic composition to produce titanium boride particles dispersed therein. The introduction of boron has been accomplished by several different methods, such as conventional cast-and-wrought processing, powder metallurgy techniques such as gas atomization, and a blended elemental approach. The first two methods suffer from the limited solubility of boron in titanium. The boron tends to segregate strongly, forming relatively large titanium boride particles that are detrimental to ductility and fatigue. In order to avoid the segregation problem, the levels of boron added to the metallic composition by these first two methods is severely restricted, usually to the hypoeutectic portion of the phase diagram, limiting the potential benefits of the boron addition, or the cooling rate during solidification must be very high. The blended elemental approach allows much larger additions of boron. However, because the boron is typically added as titanium diboride, and the phase in thermodynamic equilibrium with the alpha phase of titanium is the very-stable titanium monoboride, extended times at elevated temperatures are required to fully convert the titanium diboride to titanium monoboride. The required high temperatures and long times prevent the production of a uniform fine dispersion of titanium boride particles in the metallic composition. Additionally, fine freestanding titanium boride or titanium diboride particles tend to agglomerate, reducing the uniformity of the final product. The result of all of these production approaches is that a significant volume fraction of the titanium boride is present as large particles that are typically 10-100 micrometers in their largest dimensions. These large particles have some beneficial strengthening effects, but they are not optimal for ductility, crack initiation, and static, creep, and fatigue strength.
It has been possible, using existing melting, casting, and conversion practice, to prepare non-boron-containing titanium-base metallic composition components such as compressor and fan disks that are fully serviceable. However, there is a desire and need for a manufacturing process to produce the disks and other components with even further-improved properties arising from the presence of titanium boride particles and greater freedom from irregularities, thereby improving the operating margins of safety. The present invention fulfills this need for an improved process, and further provides related advantages.